FR3151411A1 - Head-up display device and method for controlling the intensity of a light beam - Google Patents

Abstract

A head-up display device comprises: - a housing (2); - an image generation unit (4) housed in the housing and configured to produce a light beam (L); and - an optical system housed in the housing (2) and configured to transmit the light beam (L) through a window (16) provided in the housing (2), the optical system comprising at least one element (8) having one face (9) configured to at least partly reflect the light beam (L). The element (8) is designed to transmit through itself a portion of radiation incident on said face (9). The head-up display device comprises an electronic measuring circuit (50) housed in the housing (2) and comprising a support (52), a first sensor (54) carried by the support (52) and arranged to measure an ambient brightness outside the housing (2), and a second sensor (56) carried by the support (52) and arranged to measure an intensity of said portion of radiation transmitted through said element (8). Figure for abstract: Fig. 1

Description

Head-up display device and method for controlling the intensity of a light beam

The present invention relates generally to the field of display systems, and in particular head-up display systems.

It relates more particularly to a head-up display device and a method for controlling the intensity of a light beam.

Technological background

Vehicles are increasingly being equipped with a head-up display device to project various information, generally related to driving, into the driver's field of vision so that the driver can view this information without having to look away from the road ahead.

Such a head-up display device comprises an image generating unit housed in a housing and configured to generate a light beam, and an optical system also housed in the housing and configured to transmit the light beam through a window provided in the housing.

The light beam emerging through the window is reflected on a partially transparent blade towards the driver's eyes so that the driver can see in front of the vehicle a virtual image that includes the above-mentioned information. The partially transparent blade can in practice be the windshield of the vehicle or a dedicated element, sometimes called a combiner.

Due to the light power that is required to be produced in certain operating phases of the head-up display device, the image generating unit releases a significant amount of heat inside the housing. Furthermore, in certain situations, solar radiation may enter the housing through the window and follow a path close to the reverse path to that followed by the light beam generated by the image generating unit, which results in a risk of damage to the image generating unit.

In this context, a head-up display device is proposed comprising a housing, an image generation unit housed in the housing and configured to produce a light beam, and an optical system housed in the housing and configured to transmit the light beam through a window provided in the housing, the optical system comprising at least one element, one face of which is configured to at least partly reflect the light beam, characterized in that said element is designed to transmit through said element a portion of radiation incident on said face, and in that the head-up display device comprises an electronic measuring circuit housed in the housing and comprising a support, a first sensor carried by the support and arranged to measure an ambient brightness outside the housing, and a second sensor carried by the support and arranged to measure an intensity of said portion of radiation transmitted through said element.

We thus place on the same support a first sensor which measures the ambient brightness (used for example to set up an intensity reduction strategy, or " dimming " according to the Anglo-Saxon term, as explained below) and a second sensor which measures a value representative of incident radiation on said face of the element of the optical system, which makes it possible to evaluate the presence of a solar flux entering through the window and presenting a risk of heating of the head-up display device.

The housing may comprise, for example, a side wall and an upper wall in which the window is provided; the second sensor may then be interposed between the element and the side wall.

The head-up display device may include an electronic control circuit configured to control the intensity of the light beam produced by the image generating unit based on the measured ambient brightness and said measured intensity. As indicated above, the measured ambient brightness may be used in an intensity reduction strategy; further, as explained below, said measured intensity may be used in determining a charge reduction temperature.

The electronic control circuit may for example be designed to determine an admissible intensity or brightness value as a function of said measured intensity (taking into account in the example described below the load reduction temperature determined as a function of said measured intensity), and/or to determine said intensity of the light beam produced as a function of the admissible intensity or brightness value and the measured ambient brightness.

For example, it is possible to provide for the electronic control circuit to be interposed between the image generation unit and the side wall.

The element of the optical system may be located outside a space between the window and the first sensor so as to provide a gap for the passage of light from the window to the first sensor. Thus, by measuring the intensity of this light passing through the gap provided, the first sensor effectively measures the ambient brightness outside the housing. However, as a variant, other means could be used to enable the measurement of this ambient brightness, for example a light guide placed between the window and the first sensor.

In the example described here, the first sensor and the second sensor are fixed on the same face of the support.

According to one possible embodiment, said face of said element is designed to partially transmit a portion of said radiation located in the visible spectrum.

The element may for example have an intensity reflection coefficient greater than 80% for light rays incident on said face of said element and polarized in a first direction, and/or a transmission coefficient greater than 30% for light rays incident on said face of said element and polarized in a second direction orthogonal to the first direction.

According to another possible embodiment, possibly combinable with the previous one, said face of said element is designed to transmit at least part of the infrared radiation.

Also provided is a method for controlling the intensity of a light beam produced by an image generation unit included in a head-up display also comprising a housing housing the image generation unit, and an optical system housed in the housing and configured to transmit the light beam through a window provided in the housing, the optical system comprising at least one element one face of which is configured to at least partially reflect the light beam, the method comprising the following steps:

- measurement of ambient brightness outside the housing using a first sensor carried by a support;

- measurement, by means of a second sensor carried by the support, of an intensity of a part of radiation incident on said face and transmitted through the element.

This method may further comprise a step of controlling the intensity of the light beam as a function of the measured ambient brightness and said measured intensity.

This process may also include the following steps:

- measurement of an internal temperature in the case;

- determination of a maximum admissible brightness as a function of the measured internal temperature and a load reduction temperature.

The load reduction temperature can for example be determined based on said measured intensity.

The method may comprise a step of determining an effective brightness as a function of the maximum admissible brightness and the measured ambient brightness.

Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

Brief description of the figures

The description which follows with reference to the attached drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

On the attached drawings:

represents an example of a head-up display device according to the invention; and

is a flowchart showing a possible operation for the head-up display device of the .

There represents an example of a head-up display device according to the invention.

This head-up display device (which here equips a vehicle) comprises a housing 2 in which are housed an image generation unit 4 and an optical system. This optical system here comprises a curved mirror 6 and an element 8 forming a folding mirror for the light beam generated by the image generation unit 4, as explained below.

The housing 2 comprises a top wall 10, side walls 12 and a bottom wall 14.

A window 16 (sometimes referred to as a " cover window ") is provided in the upper wall 10 to allow the passage, towards the outside of the housing 2, of the light beam generated by the image generation unit 4 and transmitted through the optical system. The window 16 is for example produced by means of a transparent part (typically made of plastic) closing an opening formed in the housing 2, here in the upper wall 10 of the housing 2.

In the example described here, the image generation unit 4 comprises at least one light source 20 and a screen 22 arranged on the path of the light rays emitted by the light source 20. The light source 20 is for example produced by means of a light-emitting diode or LED. The image generation unit 4 here further comprises a reflector 24 extending (over at least a portion of the periphery of the light source 20) between the light source 20 and the screen 22.

The screen 22 comprises a plurality of variable transmittance elements organized in the form of a matrix and each configured to transmit an adjustable proportion (by the electronic control circuit 44 mentioned below) of the light that this element receives from the light source 20.

The screen 22 is for example a liquid crystal screen (or LCD for " Liquid Crystal Display "). In this case in particular, the screen 22 comprises an input polarizer (on the face of the screen 22 facing the light source 20) and an output polarizer (on the face of the screen 22 opposite the light source 20).

The image generation unit 4 thus generates a light beam L at the output of the screen 22, specifically here a light beam emerging from the output polarizer of the screen 22. The light beam L is therefore polarized according to the polarization direction of this output polarizer.

In the example described here, the light source is carried by a first printed circuit 26. The reflector 24 can thus extend between the first printed circuit 26 and the screen 22, over at least part of the periphery of the light source 20.

The first printed circuit 26 is arranged in contact with a radiator 28 located outside the housing 2 to allow the cooling of the image generation unit 4, in particular of the circuits carried by the first printed circuit 26.

In the present example, a part of the image generation unit 4 (part formed here by the first printed circuit 26 and a proximal region of the reflector 24) passes through an opening formed in the bottom wall 14 of the housing 2. This opening can then for example be closed by the radiator 28, as visible in .

The head-up display device here further comprises internal walls 30, 32 which partly delimit an optical chamber 34 crossed by the light beam L. In the example of the , the internal walls 30, 32 comprise a first internal wall 30 and a second internal wall 32.

The first internal wall 30 extends here between a corner of the housing formed by the intersection of a side wall 12 and the bottom wall 14, and a first edge of the screen 22 directed towards the curved mirror 6. The second internal wall 32 extends here from a second edge of the screen 22 (opposite the aforementioned first edge) and a lower edge of the element 8 (i.e. an edge of the element 8 oriented towards the bottom wall 14).

In the example described, the optical chamber 34 is thus delimited by the first internal wall 30, the screen 22 (through which the beam F enters the optical chamber 34), the second internal wall 32, the element 8, the window 16 (through which the beam F exits the optical chamber 34) and the curved mirror 6.

An internal day 36 (produced here in the form of a passage) is however provided between the element 8 on the one hand and the upper wall 10 (and/or the window 16) on the other hand, in order to allow at least part of the light rays penetrating inside the housing 2 (via the window 16) to pass towards a first sensor 54 described later. Alternatively, this internal day could be produced by means of a transparent plate (possibly formed in one piece with the piece forming the window 16) extending between the element 8 and the upper wall 10.

In the example described here, the curved mirror 6 and the element 8 are arranged so that the light beam L generated by the image generation unit 4 is reflected by the element 8 in the direction of the curved mirror 6, then by the curved mirror 6 in the direction of the window 16 and a partially transparent blade 40 located in front of the user (here the driver of the vehicle), then by the partially transparent blade 40 in the direction of the eyes U of the user, as schematically represented in FIG. , so as to form a virtual image I in front of the user (on the other side of the partially transparent blade 40 relative to the user).

The partially transparent blade 40 may be the windshield of the vehicle, as is the case in the example described here, or, alternatively, a dedicated part, commonly called a " combiner ".

The element 8 thus comprises a face 9 configured to reflect a significant portion (for example more than 80% in intensity) of the incident light beam (received here directly from the image generation unit 4).

The element 8 is furthermore designed to allow the passage (i.e. to allow the transmission through the element 8) of a portion of the radiation incident on the aforementioned face 9.

When the light beam emitted by the image generation unit 4 is polarized according to a first polarization direction (as indicated above), the aforementioned face 9 of the element 8 may have an intensity reflection coefficient greater than 80% for light rays of the visible spectrum polarized according to the first polarization direction. The element 8 may, on the other hand, transmit (for example with a transmission coefficient greater than 30%) infrared radiation and/or radiation formed from light rays of the visible spectrum polarized according to a second polarization direction orthogonal to the first polarization direction.

The element 8 here comprises a transparent blade 60 and a film 62. The film 62 is for example glued to the transparent blade 60. The film 62 thus has a first face in contact with the transparent blade 60 and a second face, opposite the first face and which forms the aforementioned face 9.

The film 62 (and here in particular its second face) is designed to reflect the light rays of the visible spectrum polarized according to the first direction of polarization (already mentioned), for example with an intensity reflection coefficient greater than 80%, and to allow the other radiation to pass (with a transmission coefficient greater than 30%), namely in particular infrared radiation and/or the light rays of the visible spectrum polarized according to the second direction of polarization orthogonal to the first direction of polarization.

Film 62 is, for example, a CMF filter (for “ Cold Mirror Film ”) marketed by the company 3M.

The radiation (infrared radiation and/or light rays of the visible spectrum polarized according to the second polarization direction) which is transmitted through the film 62 is also transmitted through the transparent blade 60.

The curved mirror 6 has a non-zero optical power. Furthermore, the curved mirror 6 (precisely its reflecting surface facing the element 8) may have a shape adapted to compensate for distortions generated for example by the reflection on the partially transparent blade 40 (in particular when this partially transparent blade is the windshield of the vehicle).

The head-up display device shown in the also includes a second printed circuit 42 which carries an electronic control circuit 44.

The second printed circuit 42, and therefore the electronic control circuit 44, are here housed inside the housing 2. In the example described, the second printed circuit 42 and/or the electronic control circuit 44 is located between the element 8 (precisely the lower edge of the element 8) and the bottom wall 14. As visible in the , the second printed circuit 42 can extend parallel to a side wall 42 (this side wall 42 being here, among the different side walls, the side wall closest to the element 8).

Furthermore, in the example described here, the electronic control circuit 44 (i.e. also the second printed circuit 42) is interposed between the image generation unit 4 and the side wall 12.

As described in more detail below, the electronic control circuit 44 is notably designed to control the power of the light source 20 (i.e. to control the supply current of the light source 20 when this light source 20 is a light-emitting diode) and to control the transmittance of each of the variable transmittance elements of the screen 22 (this in order to obtain a certain brightness of the light beam L and therefore of the virtual image I).

The head-up display device further comprises an electronic measuring circuit 50 comprising a third printed circuit 52 carrying (i.e. forming a support for) a first sensor 54 and a second sensor 56.

The electronic measuring circuit 50 is housed in the housing 2.

The first sensor 54 and the second sensor 56 are here located on the same face of the third printed circuit or support 52, namely the face facing the element 8.

As visible on the , in the example described, the third printed circuit (or support) 52 extends at least partly between the element 8 and a side wall 12. The third printed circuit 52 also extends here parallel to this side wall 12.

The first sensor 54 is arranged to measure ambient brightness outside the housing 2. To do this, the first sensor 54 faces the internal gap 36 (here arranged between the element 8 and the upper wall 10 as already indicated) so that this first sensor 54 receives light rays coming from outside the housing 2 through the window 16 and the internal gap 36. In other words, the element 8 extends outside a space located between the first sensor 54 and the window 16.

The first sensor 54 is here mounted at an upper region of the printed circuit or support 52, that is to say at a region of the printed circuit or support 52 located close to the upper wall 10.

Furthermore, in the example described, the window 16 comprises a curved portion which extends at least partly in a direction (here vertical) perpendicular to the upper wall 10 (which is here essentially flat, for example horizontal).

The first sensor 54 can thus in particular receive through the window 16 light rays having a small angular deviation from the horizontal (for example an angular deviation of less than 30° in absolute value), coming for example from a region where the virtual image I is formed. Indeed, these light rays arrive (from outside the housing 2) on the window 16 with a limited angle of incidence and are therefore transmitted through the window 16.

The first sensor 54 therefore allows a relevant measurement of the ambient brightness L _AMB in the region of the virtual image I.

In the example described herein, an optical element 58 (such as a light guide) is placed in front of the first sensor 54 so that the first sensor 54 detects rays over an expanded angular range.

The second sensor 56 is arranged to measure the intensity of a portion of the radiation incident on the face 9 and transmitted through the element 8, as described above.

In the example described here, the second sensor 56 is a brightness sensor and measures the light intensity in the visible range.

During operation of the device in the absence of sunlight entering the optical chamber 34, the only light rays incident on the face 9 are those of the light beam L produced by the image generation unit 4; as explained above, due to their polarization (according to the first polarization direction), these light rays (belonging to the light beam L) are reflected (at least 80% in intensity) by the element 8 (here precisely by the film 62 of the element 8), so that the second sensor 56 measures a low intensity.

On the other hand, when a solar flux F enters the optical chamber 34 through the window 16 as shown in the and that this solar flux F is reflected by the curved mirror 6 in the direction of the element 8, the light rays included in this solar flux F and having a polarization oriented according to the second polarization direction are transmitted (with a transmission coefficient of at least 30%) through the element 8 (here precisely through the film 62 then the transparent blade 60) and produce a notable light intensity at the level of the second sensor 56.

The second sensor 56 can thus measure a value representative of the light intensity E of the solar flux F which follows a path close to the reverse path of the path taken by the light beam L, solar flux F which would therefore risk damaging the screen 22.

According to a possible variant, the second sensor could be sensitive to infrared radiation, and would then measure the infrared radiation included in the solar flux F and transmitted through the element 8.

In the example described here, an optical element 4 (such as a light guide) is placed in front of the second sensor 56 so that the second sensor 56 detects rays over an expanded angular range.

As schematically represented in the , a wired link 66 (such as a connection bus) connects for example the third printed circuit 52 and the second printed circuit 42 so as to allow an exchange of information between the electronic measurement circuit 50 and the electronic control circuit 44, and in particular so as to transmit from the electronic measurement circuit 50 to the electronic control circuit 44 the ambient brightness L _AMB measured by the first sensor 54 and/or the light intensity E measured by the second sensor 56.

The head-up display device of Fig. 1 may further be equipped with other sensors (not shown in the ), for example an ambient temperature sensor T _AMB (mounted here on the second printed circuit 42), a temperature sensor T _BKL of the at least one light source 20 (mounted here on the first printed circuit 26) and a temperature sensor T _TFT of the screen 22 (mounted here on an edge, for example the lower edge, of the screen 22). The measurements made by these other sensors are also transmitted to the electronic control circuit 44, here by wired connections (not shown). For each sensor, it is possible to provide for example between 5 and 20 acquisitions per second and/or a time filtering (low-pass in frequency) to avoid fluctuations that are too rapid in the signal concerned.

We now describe with reference to the a possible operation for the head-up display device of the .

This method is implemented here by the electronic control circuit 44 (precisely in practice by a processor of the electronic control circuit 44).

The process of the begins with an initialization step E2.

In the example described here, the following variables are initialized during this initialization step:

- a load reduction temperature T _d (or " derating " temperature according to the Anglo-Saxon term often used) is initialized at a predefined value (for example between 20°C and 80°C)

- a first corrective factor c _N and/or a second corrective factor c _J are initialized to 0.

The process of the continues with a step E4 in which the electronic control circuit 44 determines, using in particular the light intensity measured by the second sensor 56, whether the head-up display device is exposed to a solar flux which requires a protective measure.

In the example described here, during step E4, the electronic control circuit 44 evaluates the effective irradiance SL _eff as a function of the light intensity (or illumination) E measured by the second sensor 56 and determines whether a protective measure is required by comparing the effective irradiance SL _eff evaluated with an irradiance threshold SL _lim .

The electronic control circuit 44 evaluates, for example, the effective irradiance SL _eff as proportional to the luminous intensity E measured by the second sensor 56, here according to the formula:

SL _eff = (E/E _max ).SL _max

where E _max is the maximum light intensity measured by the second sensor 56 and SL _max the corresponding maximum irradiance (for example between 800 Wm ^-2 and 1200 Wm ^-2 ), these values being determined by means of experiments carried out for the relevant architecture of the head-up display device.

If the effective irradiance SL _eff is greater than (or equal to) the irradiance threshold S _lim (i.e. if it is determined that a protective measure is required because of the solar flux entering the head-up display device), the method continues to step E6.

If the effective irradiance SL _eff is (strictly) lower than the irradiance threshold S _lim (i.e. if it is determined that a protective measure is not required due to the solar flux entering the head-up display device), the method continues to step E8.

Alternatively, it is possible to determine whether the head-up display device is exposed to a solar flux that requires a protective measure by comparing (directly) the intensity measured by the second sensor 56 to an intensity threshold (the method continuing at step E6 if the intensity measured by the second sensor 56 is greater than or equal to the intensity threshold, and at step E8 if the intensity measured by the second sensor 56 is strictly less than the intensity threshold).

In step E6, the electronic control circuit 44 sets the first corrective factor c _N to a new value as a function of the ambient temperature T _AMB and the effective irradiance SL _eff .

The first corrective factor c _N therefore depends here on the light intensity measured by the second sensor 56.

In practice, the first corrective factor c _N can be determined (by the electronic control circuit 44) by reading in a first correspondence table. This first correspondence table is for example stored in a memory of the electronic control circuit 44. This first correspondence table can then list a plurality of possible values of the first corrective factor in association respectively with different pairs each defined by an ambient temperature value and an effective irradiance value (or, as a variant, in association respectively with different pairs each defined by an ambient temperature value and a light intensity value possibly measured by the second sensor 56).

After step E6, the method continues with step E8 described now.

In step E8, the electronic control circuit 44 determines the second corrective factor c _J as a function of at least one temperature measured at the image generation unit 4.

For example, two temperatures measured at the image generation unit 4 are used here, namely the temperature T _BKL of the at least one light source 20 and the temperature T _TFT of the screen 22 mentioned above.

The electronic control circuit 44 therefore determines here the second corrective factor c _J as a function of the temperature T _BKL of the at least one light source 20 and the temperature T _TFT of the screen 22.

In practice, the second corrective factor c _J can be determined (by the electronic control circuit 44) by reading in a second correspondence table. This second correspondence table is for example stored in a memory of the electronic control circuit 44. It can be provided that, for certain pairs defined by a value of the temperature T _BKL of the at least one light source 20 and a value of the temperature T _TFT of the screen 22 (non-problematic values), the second corrective factor c _J is zero.

The process of the then continues with a step E10 of updating the load reduction temperature T _d as a function of the first corrective factor c _N and the second corrective factor c _J .

The load reduction temperature T _d can therefore be updated (indirectly, via the first corrective factor c _N ) as a function of the intensity measured by the second sensor 56.

In the example described here, the load reduction temperature T _d is updated by subtracting from it the maximum value between the first correction factor c _J and the second correction factor c _N , which can be written as: T _d ← T _d - max(c _J , c _N ), where max is the function which gives the maximum value among the two values given as argument.

Of course, when the first corrective factor c _J and the second corrective factor c _N are zero, the load reduction temperature T _d is unchanged during step E10.

The process of the then includes a step E12 of determining a maximum admissible brightness X _adm as a function of an internal temperature in the housing (here the ambient temperature T _AMB ) and the load reduction temperature T _d .

In the example described here, predefined values are used for this purpose: a minimum operating temperature T _min (for example between -45°C and 0°C), a maximum operating temperature T _max (for example between 80°C and 120°C), a minimum brightness X _min (for example between 0% and 1%) and a maximum brightness X _max (for example 100%).

The electronic control circuit 44 can then determine the maximum admissible brightness X _adm according to the following rules:

- if the ambient temperature T _AMB is (strictly) lower than the minimum operating temperature T _min , the maximum admissible brightness X _adm is set to the minimum brightness X _min ;

- if the ambient temperature T _AMB is higher (or equal) than the minimum operating temperature and lower (or equal) to the load reduction temperature T _d (as determined after updating in step E10), the maximum admissible brightness X _adm is set to the maximum brightness X _max ;

- if the ambient temperature T _AMB is higher (strictly) than the load reduction temperature T _d and lower (or equal) to the maximum operating temperature T _max , the maximum admissible brightness X _adm is set to X _max -(X _max -X _min ).(T _AMB -T _d )/(T _max -T _d );

- if the ambient temperature T _AMB is higher (strictly) than the maximum operating temperature Tmax, the maximum admissible brightness X _adm is set to the minimum brightness X _min .

In other words, when the ambient temperature T _AMB is between the load reduction temperature T _d and the maximum operating temperature T _max , the maximum admissible brightness X _adm varies linearly as a function of the ambient temperature T _AMB (being X _max when T _AMB = T _d and X _min when T _AMB = T _max ).

Since the charge reduction temperature T _d can vary depending on the light intensity measured by the second sensor 56, the maximum admissible brightness X _max can also vary depending on this light intensity measured by the second sensor 56.

The process of the then comprises a step E14 of determining the effective brightness X _eff (or brightness to be used) as a function of the maximum admissible brightness X _adm (determined in step E12), the ambient brightness L _AMB measured by the first sensor 54 and the brightness X _u requested by the user.

The electronic control circuit 44 determines, for example, the effective brightness X _eff as a function of the ambient brightness L _AMB and the requested brightness X _u according to an intensity reduction strategy (or " dimming " strategy according to the commonly used Anglo-Saxon term) without, however, exceeding the maximum admissible brightness value X _adm .

The intensity reduction strategy aims to avoid glare of the driver by taking into account the ambient brightness L _AMB .

If we denote g as the function defining the intensity reduction strategy, that is to say by writing g(X _u ,L _AMB ) the luminosity determined according to this strategy, the electronic control circuit 44 can then determine the effective luminosity X _eff as follows:

X _eff = min(g(X _u ,L _AMB ),X _adm ) where min is the function which gives the minimum value among the two values given as argument.

The control circuit 44 can then control the image generation unit 4 in step E16 so as to produce a light beam L having a light intensity corresponding to the effective brightness X _eff determined in step E14.

According to one possible embodiment, when the light source 20 is produced by (at least) one light-emitting diode, the control circuit 44 determines the intensity of the current in the light-emitting diodes as a function of the temperature T _BKL of the at least one light source 20, controls the injection into the at least one light-emitting diode of a current having the intensity thus determined, then controls the transmittance of the variable transmittance elements of the screen 22 as a function of the brightness X _eff to be obtained, and of the determined intensity (and, of course, for each element, of a value which defines the appearance of the pixel concerned in the image to be displayed).

Claims (14)

Head-up display device comprising:
- a housing (2);
- an image generation unit (4) housed in the housing and configured to produce a light beam (L); and
- an optical system housed in the housing (2) and configured to transmit the light beam (L) through a window (16) provided in the housing (2), the optical system comprising at least one element (8) one face (9) of which is configured to at least partly reflect the light beam (L),
characterized in that said element (8) is adapted to transmit through said element (8) a part of radiation incident on said face (9); and
in that the head-up display device comprises an electronic measuring circuit (50) housed in the housing (2) and comprising a support (52), a first sensor (54) carried by the support (52) and arranged to measure an ambient brightness outside the housing (2), and a second sensor (56) carried by the support (52) and arranged to measure an intensity of said portion of radiation transmitted through said element (8). A head-up display device according to claim 1, wherein the housing (2) comprises a side wall (12) and an upper wall (10) in which the window (16) is provided, the second sensor (56) being interposed between the element (8) and the side wall (12). Head-up display device according to claim 1 or 2, comprising an electronic control circuit (44) adapted to control the intensity of the light beam (L) produced by the image generation unit (4) as a function of the measured ambient brightness and of said measured intensity. A head-up display device according to claim 3, wherein the electronic control circuit (44) is designed to determine an admissible intensity or brightness value as a function of said measured intensity, and to determine said intensity of the light beam produced (L) as a function of the admissible intensity or brightness value and the measured ambient brightness. Head-up display device according to claim 3 or 4 taken in dependence on claim 2, in which the electronic control circuit (44) is interposed between the image generation unit (4) and the side wall (22). A head-up display device according to one of claims 1 to 5, wherein the element (8) is located outside a space between the window (16) and the first sensor (54) so as to provide a gap (36) for the passage of light from the window (16) to the first sensor (54). Head-up display device according to one of claims 1 to 6, in which the first sensor (54) and the second sensor (56) are fixed on the same face of the support (52). Head-up display device according to one of claims 1 to 7, wherein said face (9) of said element (8) is designed to partially transmit a portion of said radiation located in the visible spectrum. Head-up display device according to one of claims 1 to 8, in which the element (8) has an intensity reflection coefficient greater than 80% for light rays incident on said face (9) of said element (8) and polarized in a first direction, and a transmission coefficient greater than 30% for light rays incident on said face (9) of said element (8) and polarized in a second direction orthogonal to the first direction. Method for controlling the intensity of a light beam (L) produced by an image generation unit (4) included in a head-up display also comprising a housing (2) housing the image generation unit (4), and an optical system housed in the housing and configured to transmit the light beam (L) through a window (16) provided in the housing (2), the optical system comprising at least one element (8) one face (9) of which is configured to at least partly reflect the light beam (L), the method comprising the following steps:
- measuring ambient brightness outside the housing by means of a first sensor (54) carried by a support (52);
- measuring, by means of a second sensor (56) carried by the support (52), an intensity of a part of radiation incident on said face (9) and transmitted through the element (8). Method according to claim 10, comprising a step of controlling the intensity of the light beam (L) as a function of the measured ambient brightness and of said measured intensity. A method according to claim 10 or 11, comprising the following steps:
- measurement of an internal temperature in the housing (2);
- determination of a maximum admissible luminosity (X _adm ) as a function of the measured internal temperature and a load reduction temperature (T _d ). A method according to claim 12, wherein the charge reduction temperature (T _d ) is determined as a function of said measured intensity. Method according to claim 12 or 13, comprising a step of determining an effective brightness (X _eff ) as a function of the maximum admissible brightness (X _adm ) and the measured ambient brightness.